They are polygamous.

They don’t actually ‘keep’ their mates. They find another Ghidorah, engage in the courtship battle to determine which will be the male and which will be the female (a violent courtship for a violent species), they copulate, and go their separate ways afterwards and probably won’t see each other ever again. The pregnant Ghidorah can no longer mate for a while until they lay the eggs, but other than that, both Ghidorah will mate with whatever other full-grown Ghidorah they come across if they ever get the opportunity.

There is no emotional attachment to their coupling, only instinct and lust.

They’re not really picky on their mates; they can’t afford to be. In the vastness of space, running into another Ghidorah is super rare and pickiness is not an ideal trait to have. Who knows, you probably won’t get another chance to mate again for millions of years! As long as they’re sexually mature, they dun care. They both made it this far in life, they’re gonna smash.

But anyway, there may be some difficulty and objection outside the norm on rare occasions, like with our particular Ghidorah. San doesn’t really care if they’re the one with eggs; he gets laid either way! But Ichi and Ni would sooner try to kill their potential mate than be the pregnant dragon. In this case, San just goes along with whatever his older brothers do. But in most cases, the mating process between Ghidorah specimens goes without a hitch. Violent and bloody as is natural for their kind, sure, but consensual.